Text (SA): Whenever you gain life, you may pay {1}{W}. If you do, put a +1/+1 counter on target creature for each 1 life you gained.
You choose a target when the triggered ability is put on the stack. You choose whether or not to pay during resolution. [Shards of Alara FAQ 2008/09/23]
Triggers once on each life gaining event, regardless of how much life is gained in that event. So if you gain 3 life in one event, you pay once to get three +1/+1 counters. [Shards of Alara FAQ 2008/09/23]
